The official Twitter account of Mana series, or Seiken Densetsu as it is known in Japan, has posted what appears to be a 10-second clip of Seiken Densetsu 3 running on a Nintendo Switch.
— 聖剣伝説 -公式- (@Seiken_PR) March 20, 2017
The game originally released for Super Famicom (SNES) in Japan on the 30th September 1995. It never made its way to the Western markets, although you can still find copies on import. It also never saw a release on the 3DS, Wii, or Wii U Virtual Console.
Square Enix is yet to officially announced a Nintendo Switch version of Seiken Densetsu 3 so it may just be a proof of concept. Then again, why would you Tweet showing the game running on new hardware if you didn’t plan on releasing it.